Bone Plate System

ABSTRACT

A bone plate system is provided that includes bone plates and bone anchor assemblies for being inserted into bone plate bores to secure the bone plates to one or more bones. In one aspect, the bone plate system includes a bone anchor assembly having a locking cap and a locking fastener connected to a head portion of a bone anchor. The bone anchor assembly is driven into a bone plate bore before the locking fastener is shifted to expand the locking cap and fix the bone anchor assembly within the bore. In another aspect, the bone plate system includes a bone anchor assembly having a locking cap that deflects radially inward due to contact with the bone plate at large bone anchor assembly insertion angles. In addition, a bone anchor assembly having a preassembled condition with a locking cap resisting back out of a locking fastener.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/047,926, filed Apr. 25, 2008, which is hereby incorporated byreference as if fully set forth herein.

FIELD OF THE INVENTION

The invention relates to a bone plate system and, more particularly, toa bone plate system that allows for polyaxial bone anchor insertion andrigid fixation of the bone anchor relative to a bone plate.

BACKGROUND OF THE INVENTION

There are presently many different types of plate and fixture systemsfor securing two or more bones or bone fragments in relative position sothat the bones may fuse or heal, or so that tissue adjacent the bonesmay heal without disruption from the movement of the secured bones. Asused herein, the term bone may refer to a bone, or a bone fragment orportion, and the term may refer to a portion of a bone that is coveredwith another material, such as the endplates covering the top and bottomsurface of a vertebra. Also as used herein, the term fusion refers tothe joining of materials, such as bone or graft material, and the fusionsite is the entire region in which fusion may be desired. As isapparent, bone plate systems may be used to fuse a variety of differenttypes of bone, including spinal vertebrae.

The bones and connective tissues of a normal human spinal column includevertebrae that support the weight of the human body as well as protectthe spinal cord and other elements of the nervous system. A typicalvertebra consists of a vertebral body and a vertebral arch positionedposterior to the vertebral body. The vertebral body is generally alignedwith the vertebral bodies of the adjacent vertebrae to transmit loadingforces along the spinal column. The vertebral arch, on the other hand,is formed by a pair of pedicles and a pair of laminae that surround andprotect the spinal cord.

The vertebrae of the spine are classified into five regions based ontheir position along the spinal column: cervical, thoracic, lumbar,sacral, and coccygeal (tail bone). The cervical region contains sevensmall vertebrae located near the neck and base of the skull, while thethoracic region contains twelve larger vertebrae near the rib cage thatincrease in size going downward along the spine. The lumbar region isgenerally in the lower back area and has five vertebrae that are largerthan the other vertebrae in the spinal column in order to support thefull weight of the upper torso. Normally, the cervical, thoracic, andlumbar vertebrae have intervertebral discs positioned between adjacentvertebral bodies that allow slight movement of the vertebrae and dampenjarring forces which act on the vertebrae.

The sacral and coccygeal vertebrae, on the other hand, are fused inmaturity and lack intervertebral discs. More specifically, the sacralregion consists of five vertebrae fused together to form the sacrum, alarge triangular bone positioned between the hip bones. The sacrum isrelatively concave and faces inward to the body such that the firstsacral vertebra (S1) extends obliquely relative to the last lumbarvertebra (L5). Additionally, the anterior-most point of the S1 vertebraincludes a formation referred to as the sacral promontory positionedadjacent the intervertebral disc located between the S1 and L5vertebrae.

Various types of injury or deformity may affect the structure andfunction of the spinal column. For example, sudden trauma may breakbones or tear soft tissues which support the spine. Alternatively,genetic disorders, tumors, or infections may cause bones to deteriorate.Further, excessive movement of one vertebral body relative to anothermay cause compression of the spinal cord or nerves. Patients who sufferfrom one or more of these conditions often experience extreme anddebilitating pain, and may incur permanent neurologic damage if theconditions are not properly treated.

One technique for treating a number of spinal disorders is known asinterbody spinal fusion. This procedure involves removing anintervertebral disc and replacing it with a bone graft in conjunctionwith stabilizing the vertebrae on either side of the intervertebraldisc. The vertebrae eventually fuse together by growth of the boneacross the disc space which rigidly connects the vertebrae. Thestabilization of the vertebrae is often assisted by a surgicallyimplanted bone plate and anchor system that holds the vertebral bodiesin proper alignment and allows the bones to heal. Such techniques havebeen successfully used to reduce pain and restore the structuralintegrity of the affected bones.

Different applications of the bone plate may present differentrequirements for bone plate systems. For example, in some instances itmay be desirable to utilize a bone plate system that limitspost-operative subsidence of the vertebrae. More specifically, vertebraemay require a higher level of strength and stability to ensure fusion ofthe affected vertebrae. A bone plate system applied to the vertebraeunder these conditions may rigidly fix the bone plate system and theaffected vertebrae in the desired orientation to provide the requiredbiomechanical stability while fusion is achieved.

In some instances, it is also highly desirable for a bone plate systemto provide a bone anchor that may be driven into bone at a variety ofdifferent insertion angles. This functionality provides a surgeon withthe flexibility to respond to anticipated or unanticipated features ofthe subject anatomy. Further, the ability to polyaxially drive the boneanchor into a vertebra also permits the surgeon to angle the bone anchoraway from the end plates of the vertebral body. Engaging the bone anchorwith the end plate in some instances is undesirable as the bone anchormay cause a weakened end plate to fracture or otherwise reduce the endplate's load bearing capacity.

Many prior bone plate systems fail to provide the desired functionalityand ease of use required by a number of bone fusion procedures. One suchshortcoming is that many bone plate systems fail to resist backing outor loosening of the bone anchors, which are often bone screws. If thebone screws loosen, the bones are not properly secured and may beallowed to move relative to one another in an uncontrolled manner. Thismay compromise the ability to achieve optimal bone fusion and bonealignment, and it may lead to loss of graft material and damage or lossof bone. Furthermore, when the plate is located in the lumbar or sacralregions of the spinal column with heavy loading and movement of thevertebrae, these issues may be further compounded or exacerbated.Additionally, in the case of anterior lumbar or anterior sacral plates,a bone screw backing out could cause irritation or a puncture wound tointernal organs, or even damage to the circulatory or nervous systems.

Some prior bone plate systems seek to provide polyaxial insertion andfixation of a bone screw using a set screw that expands the bone screwhead. In these systems, the bone screw is passed through a bore formedin a bone plate until the bone screw head is positioned within the bore.The set screw is inserted into the bone screw head to expand the headuntil the head presses against the walls of the bore such that the bonescrew is held in place within the bore by the expanded head. However, topermit the bone screw head to expand, the head is constructed to beflexible for this purpose. For example, in U.S. Pat. No. 6,575,975 toBrace et al., the bone screw head is weakened by removing material whichpermits portions of the head to flex outwardly. When the bone plate islocated in the lumbar or sacral region of the spinal column, thisapproach is undesirable because weakening the bone screw head may causethe head material to yield or otherwise deform when subjected to loadingof the vertebra. Failure or deformation of the bone screw head may alsocompromise the stability of the bone plate system which, in turn, couldcause the bone fusion procedure to fail. Another problem with theseconfigurations is that the stress on the system may lead to the setscrew backing out over time.

Other bone plate systems fix a bone screw within the bore of a boneplate using an annular bushing located within the bore before the bonescrew is inserted therethrough. The bone screw is then inserted throughan opening in the bushing until the bone screw head is positioned withinthe bushing. The bone screw head may have an outer profile that causesthe bushing to expand into tight engagement with the bore surface. Inprior bone plates, the movement of the bushing is limited to theconfines of the bore in which the bushing is received. As is apparent,the angle at which the bone screw is inserted through the bore and intoa bone is similarly limited by the limited movement of the bushingwithin the bone plate bore. Further, the insertion angle of the bonescrew may also be limited by contact between the bone screw and the boneplate as the bone screw is driven into the bone plate bore.Specifically, the bone screw head may catch on the upper lip of the boneplate extending about the bore and resist passage into the bore, suchthat a different insertion angle may be required.

One problem with bushings that are placed into bone plate bores is theassembly time this requires. Another problem with this arrangement isthat the surgeon has to align the bone screw with the opening in thebushing before passing the bone screw therethrough. This complicates theprocess of fastening the bone plate onto the bones and increases theduration of the surgical procedure. In addition, one approach asdisclosed in U.S. Pat. No. 6,030,389 to Wagner et al. involves aseparate step of screwing a set screw into an anchor head to expand thebone anchor head and fix the anchor relative to the bone plate. The setscrew is necessarily larger than the bone anchor head in order to expanda bushing positioned within the bore. Not only does the large set screwrequire time-intensive installation, but it also extends a significantdistance above the head of the bone anchor such that a thicker boneplate is required to limit the distance the set screw projects into thebody.

SUMMARY OF THE INVENTION

In one aspect of the invention, a bone plate system is provided whichincludes a bone plate and a through bore having an annular surfaceextending about the through bore. The bone plate system also includes abone anchor for extending through the bone plate bore and driving into abone, the bone anchor having upper and lower ends and a longitudinalaxis extending therebetween. The bone anchor has a rigid upper headportion with a central axial bore including an enlarged diameter upperopening portion thereof that opens to the bone anchor upper end andextends axially downward therefrom. The bone plate system also includesa locking cap carried on the anchor head portion and a locking fastenerconfigured to be inserted into the bone anchor opening. The lockingfastener shifts the locking cap into tight engagement with the throughbore annular surface to keep the anchor fixed relative to the boneplate. Additionally, an upper head portion of the locking fastener isconfigured for driving the locking fastener into the axial bore of thebone anchor and is sized to be received in the upper opening portion ofthe bone anchor bore.

With respect to the locking cap, an annular wall portion of the lockingcap extends axially along the bone anchor longitudinal axis and aboutthe bone anchor head portion when the locking cap is carried on theanchor head portion. The annular wall portion includes a radially outerupper end portion configured to engage the bore annular surface and aradially inner lower end portion for engaging the bone anchor. Thelocking cap also includes a radially inner upper cam surface of thelocking cap that cooperates with the locking fastener. As the lockingfastener is inserted in the bone anchor opening, the locking fastenercams against the locking cap to deflect the annular wall portion. Inthis manner, the radially outer upper end portion shifts radiallyoutward into tight engagement with the bore annular surface while theradially inner lower end portion shifts radially inward into tightengagement with the bone anchor.

In another form of the present invention, a bone plate system isprovided including a bone plate having a through bore and a central axisextending therethrough. The bone plate system also includes an elongatebone anchor having a longitudinal axis with the bone anchor beingconfigured for polyaxial insertion through the bone plate bore. The boneanchor includes an upper head portion that carries a locking cap and isconfigured to be received within the bone plate bore. The locking caphas an annular wall portion of the locking cap that extends axiallyalong the bone anchor longitudinal axis and about the bone anchor upperhead portion. The locking cap also has a plurality of projections thatextend radially inward from the annular wall portion. A set screw isthreadingly engaged to the anchor head portion and has a narrow portionthat is aligned with the locking cap projections when the set screw isthreadingly engaged to the anchor head portion at a predetermined axialposition. Further, the set screw narrow portion is spaced from radiallyinner ends of the locking cap projections when the set screw is in thepredetermined axial position. The spacing between the projection innerends and the set screw permits the projections to shift radially inwarddue to contact of the locking cap annular wall portion with the boneplate during insertion of the bone anchor through the through bore. Thisway, the bone anchor may be inserted into the through bore at largeangles between the bone anchor longitudinal axis and the bore centralaxis without the annular wall portion getting caught on the bone plate.

In another aspect of the invention, a bone anchor assembly is providedincluding an elongate bone anchor having a head portion and a lockingcap carried on the anchor head portion. The locking cap has a pluralityof projections extending over the anchor head portion that havesubstantially flat lower surfaces which face the anchor head portion.The bone anchor assembly also includes a locking fastener having anannular collar portion that is received within the anchor head portion.Cooperating cam surfaces of the locking cap projections and the lockingfastener collar portion shift the locking cap projections radiallyoutward as the locking fastener is driven into the anchor head portion.Further, the annular collar portion of the locking fastener has asubstantially flat upper surface that is arranged below and inoverlapping confronting relation with the substantially flat lowersurfaces of the locking cap projections when the annular collar isadvanced axially beyond the locking cap projections. In this way, theflat confronting surfaces of the annular collar portion and the lockingcap projections resist locking fastener back out and maintain theanchor, locking cap, and locking fastener in a preassembledconfiguration.

A method of producing a bone anchor assembly is also provided andincludes passing an open end of a locking cap over a head portion of abone anchor and engaging surfaces of the locking cap and anchor headportion to retain the locking cap on the anchor head portion. The methodalso includes connecting a fastener to the bone anchor and passing anannular collar of the fastener beyond projections of the locking capsuch that the projections retain the fastener on the anchor head. Inthis form, the bone anchor is ready to be driven through a bone platebore and into a bone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a bone plate system in accordance with thepresent invention showing a bone plate fastened to vertebral bones;

FIG. 2 is a perspective view of the bone plate system showing the boneplate and bone anchor assemblies received in through bores of the boneplate;

FIG. 3 is an end elevational view of the bone plate system showing arange of insertion angles of bone anchors in the bone plate throughbores;

FIG. 4 is a side elevational view of the bone plate system showing therange of bone anchor assembly insertion angles;

FIG. 5 is a plan view of the bone plate;

FIG. 6 is a cross sectional view taken across line 6-6 of FIG. 5 showingannular surfaces of the bone plate through bores;

FIG. 7 is a cross sectional view taken across line 7-7 of FIG. 5 showingupper and lower chamfered surfaces of the bone plate through bores;

FIG. 8 is a perspective view of one of the bone anchor assemblies ofFIG. 2 showing a locking cap and locking fastener connected to a headportion of a bone anchor;

FIG. 9 is an elevational view of the bone anchor assembly of FIG. 8showing alignment between a slot of the locking cap and a depression ofthe bone anchor head portion;

FIG. 10 is a plan view of the bone anchor assembly of FIG. 8 showingradial projections of the locking cap;

FIG. 11 is a perspective view of the bone anchor of FIG. 8 showing anupwardly opening bore extending axially downward through the upper headportion of the bone anchor;

FIG. 12 is a side elevational view of the bone anchor of FIG. 8 showingan annular channel formed in the bone anchor head portion;

FIG. 13 is a perspective view of the locking cap of FIG. 8 showing alower inner rim projection of the locking cap;

FIG. 14 is a plan view of the locking cap of FIG. 8 showing projectionsof the locking cap that extend radially inward;

FIG. 15 is a perspective view of a locking fastener of the bone anchorassembly of FIG. 8 showing an enlarged upper head portion and dependingthreaded shank portion;

FIG. 16 is a side elevational view of the locking fastener of FIG. 15showing an annular collar between the head and neck portion;

FIG. 17 is a plan view of the locking fastener of FIG. 8 showing a boreof the locking fastener configured to receive a driver;

FIGS. 18-24 are cross-sectional views of the bone anchor assembly ofFIG. 8 that illustrate the assembly of the locking cap and lockingfastener to the bone anchor;

FIGS. 25-31 are cross-sectional views of the bone plate system showingthe bone anchor assembly being driven through one of the bone platethrough bores and the locking cap being radially expanded to fix thebone anchor assembly to the bone plate;

FIG. 32 is a perspective view of another bone plate system in accordancewith the present invention including a bone plate and bone anchorassemblies;

FIG. 33 is a plan view of the bone plate of FIG. 32 showing sidewalls ofthe bone plate through bores;

FIG. 34 is a cross-sectional view taken across line 34-34 of FIG. 33showing an annular surface of the bone plate through bores;

FIG. 35 is a cross-sectional view taken across line 35-35 of FIG. 33showing upper and lower chamfered surfaces of the bone plate throughbores;

FIG. 36 is a cross-sectional view of an alternative embodiment of anupper lip of a bone plate bore;

FIG. 37 is a perspective view of another bone plate system in accordancewith the present invention including a bone plate and bone anchorassemblies;

FIG. 38 is a perspective view of the bone plate of FIG. 37 showing alower projection at one end of the bone plate;

FIG. 39 is a side elevational view of the bone plate of FIG. 37 showingthe bone plate mounted on anterior surfaces of S1 and L5 vertebrae;

FIG. 40 is a plan view of the bone plate of FIG. 37 showing sidewalls ofthe bone plate through bores;

FIG. 41 is a cross-sectional view taken across line 41-41 of FIG. 40showing undercuts of the lower projection;

FIG. 42 is a cross-sectional view taken across line 42-42 of FIG. 40showing an annular surface and upper and lower chamfered surfaces of thebone plate through bores;

FIG. 43 is an elevational view of one of the bone anchor assemblies ofFIG. 37 including a bone anchor, a locking cap, and a locking fastener;

FIG. 44 is a perspective view of the bone anchor assembly of FIG. 43showing projections of the locking cap received between bosses of a boneanchor head portion of the bone anchor;

FIG. 45 is a perspective view of the bone anchor of FIG. 43 showing abore extending axially through a portion of the bone anchor;

FIG. 46 is an elevational view of the bone anchor of FIG. 43 showing achannel formed in the bone anchor head portion;

FIG. 47 is a plan view of the locking cap of FIG. 43 showing radiallyinner ends of the locking cap projections;

FIG. 48 is a perspective view of the locking cap of FIG. 43 showing aradially inner lower end portion of the locking cap;

FIG. 49 is an elevational view of the locking fastener of FIG. 43showing an annular collar and narrow portion of the locking fastener;

FIG. 50 is a perspective view of the locking fastener of FIG. 43 showingradially outer cam surfaces of the locking fastener;

FIG. 51 is a plan view of the locking fastener of FIG. 43 showing a boreconfigured to receive a driver;

FIG. 52 is a cross sectional view of the bone anchor assembly and boneplate of FIG. 37 showing a locking cap annular wall portion contactingthe bone plate and shifting a radially inner upper cam surface of thelocking cap radially inward;

FIG. 53 is an enlarged view of FIG. 52 showing the contact between anupper lip of the bone plate through bore and the locking cap annularwall portion;

FIG. 54 is a perspective view of another bone plate system in accordancewith the present invention including a bone plate and bone anchorassemblies;

FIG. 55 is a side elevational view of the bone plate system of FIG. 54showing the bone plate positioned on the corners of two vertebral bodiesand partially extending into the intervertebral space;

FIG. 56 is a front view of the bone plate of FIG. 54 showing an arcuatesurface on the underside of the bone plate;

FIG. 57 is a bottom view of the bone plate of FIG. 54 showing arcuatesurfaces extending away from a lower surface of the bone plate;

FIG. 58 is a plan view of the bone plate of FIG. 54 showing sidewalls ofthe bone plate through bores;

FIG. 59 is a cross-sectional view taken along line 59-59 in FIG. 58showing a plurality of surfaces that interact with the bone anchorassembly;

FIG. 60 is a cross-sectional view taken along line 60-60 in FIG. 58showing lower bore surfaces that cooperate with a cylindrical outersurface of the respective bone anchor assembly;

FIG. 61 is a side elevational view of a bone assembly of FIG. 54 showinga locking cap and a locking fastener connected to a bone anchor headportion;

FIG. 62 is a perspective view of the locking cap of FIG. 61 showingprojections that extend radially inward from an outer ring;

FIG. 63 is a perspective view of the bone anchor of FIG. 61 showing aplurality of projections extending axially upward from an upper radiallyextending portion of the anchor head;

FIG. 64 is a side elevational view of the locking fastener of FIG. 61showing an annular collar, narrow portion, and radially outer camsurfaces of the locking fastener;

FIG. 65 is a cross sectional view of the bone plate and locking fastenerof FIG. 54 showing the bone anchor assembly seated within the bone platethrough bore;

FIG. 66 is a cross sectional view similar to FIG. 65 showing the lockingfastener axially shifted into bone anchor to expand the locking collarand fix the bone anchor assembly within the bone plate through bore;

FIG. 67 is a perspective view of a bone anchor driver tool that may beused with the bone plate system;

FIG. 68 is a perspective view of a final driver that may be used withthe bone plate system;

FIG. 69 is a perspective view of a threaded pin inserter that may beused with the bone plate system;

FIG. 70 is a schematic view of a threaded pin, pin cap, and a pincentering tip showing the assembly thereof; and

FIG. 71 is a cross-sectional view of the threaded pin, pin cap, and pincentering tip of FIG. 70 showing the threaded pin driven from the pincap along the interior of the pin centering tip and into engagement witha bone plate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially to FIG. 1, a bone plate system 10 is depicted shownconnected on the anterior surface of adjacent lumbar vertebrae 12 thatare spaced by an intervertebral disc 14. In general, the bone platesystem 10 may be used to secure one or more bones in a desired spatialrelationship to aid in their healing. The bone plate system 10 may alsobe used to assist in the healing necessary after trauma or degenerativedisorders have affected the intervertebral disc 14. Furthermore, thebone plate system 10 may be used to correct and/or relieve a variety ofspinal disorders, including but not limited to degenerative disorders,disorders induced by trauma, and pinched nerves.

Turning to FIG. 2, the bone plate system 10 is shown having a bone plate100 and bone anchor assemblies 400 a-400 d. The bone plate 100 isgenerally configured to be placed on one or more bones before boneanchor assemblies 400 are inserted into through bores 102, 104, 106, 108which extend between upper and lower surfaces 110, 112 of the bone plate100. More specifically, bone system 10 permits a bone anchor assembly400 to be polyaxially inserted into one of the bores 102, 104, 106, and108 at a range of angles relative to the bone plate 100. In this manner,a surgeon may drive the bone anchor assembly through a bore of the boneplate 100 and into an underlying bone at a predetermined angle. Once thebone anchor assembly 400 is positioned within the bone anchor bore, alocking fastener 600 of the bone anchor assembly 400 is axially shiftedto expand a locking cap 500 and fix the bone anchor assembly 400 withinthe bore.

An outer sidewall 116 extends around the periphery of the bone plate 100and generally extends between the upper surface 110 and lower surface112. The bone plate 100 has a generally dog-bone shape with a narrowmiddle portion 118 and enlarged opposing ends 120, 122. The dog-boneshape reduces the overall footprint of the bone plate system 10 on thebone and provides sidewall portions 124, 126 that may be more easilygrasped with surgical tools as needed during surgery.

The bone plate 100 has several features which improve the ease withwhich the bone plate 100 may be installed onto vertebrae 12. First, thebone plate 100 has double-lobed tool slot 128 for receiving a plateinsertion tool (not pictured). In a preferred embodiment, the tool slot128 is configured to receive an expanding tip of the plate insertiontool which temporarily holds the bone plate 100 on the plate insertiontool. The bone plate 100 may then be positioned onto one or more bones,such as the vertebrae 12, using the plate insertion tool and secured tothe vertebrae 12 by way of bone anchor assemblies 400.

Second, the bone plate 100 has notches 130, 132 formed by arcuatedepressions in sidewall 116. The notches 130, 132 are sized to accepttemporary holding pins (not shown) that hold the bone plate 100 onto oneor more bones. For example, once the bone plate 100 has been positionedon the vertebrae 12, a temporary holding pin, such as a push pin, isplaced in close abutting relation to both notches 130, 132 totemporarily secure the bone plate 100 to the vertebrae 12.

As shown in FIGS. 3 and 4, the bone plate 100 may be provided with acurvature in the longitudinal and lateral directions so that the boneplate may conform to the natural curvature of the bone, as well asreduce interference with surrounding tissues. Additionally, the sidewall116 may include a flat outer surface 134 and curved upper and lowersurfaces 136, 138 which provide a smooth transition between the flatouter surface 134 and the upper and lower surfaces 110, 112. The curvedupper and lower surfaces 136, 138 also tend to reduce interference ofthe bone plate 100 with surrounding tissues.

As previously mentioned, the bone plate system 10 permits a bone anchorassembly 400 to be driven into one of the bone plate bores 102, 104,106, 108 at an angle inclined relative to the bone plate upper surface110. Driving the bone anchor assembly 400 at an angle relative to theplate may be desirable for a number of reasons, including to secure thebone anchor assembly 400 to a stronger section of the bone. Toillustrate the range of potential bone anchor angles in a lateraldirection relative to the bone plate 110, FIG. 3 discloses axes a thatextend along a respective bore central axis. The bone anchor assemblies400 a, 400 b may be driven through the respective bore 102, 104 atangles along the lateral width of the bone plate 110 that are ±Φrelative to the axis α. Conversely, FIG. 4 shows a range of bone anchorassembly 400 insertion angles along the longitudinal length of the boneplate 100 that are ±Φ from the normal axes α. In a preferred embodiment,the bone plate system 10 permits a range of bone anchor assembly 400insertion angles that are ±5° in the lateral direction and ±20° in thelongitudinal direction. To provide the polyaxial insertion capability,the bores 102, 104, 106, and 108 are configured to receive a bone anchorassembly driving end 402, shown in FIG. 8, at a variety of positionswithin the bores 102, 104, 106, and 108 such that the range of positionsfor an anchor shank 404 is generally eliptical.

Specifically, the bone plate 100 has bore sidewalls 150 a, 150 b, 150 c,and 150 d that extend between the upper and lower surfaces 110, 112 ofthe bone plate 100 and which generally define the bores 102, 104, 106,and 108, as shown in FIGS. 5-7. Although only four bores are shown, thebone plate 100 may have any number of bores as desired to adequatelysecure the bone plate 100 to bones using anchor assemblies 400. The boresidewalls 150 each have a number of interior bore surfaces, including anupper chamfer 152, an annular surface 154, and a lower chamfer 156(designated by letters a, b, c, or d for the corresponding annularsurface). The annular surface 154 generally defines a spherical pocketfor receiving the head portion 402 of the bone anchor assembly 400, thedriving end 402 expanding into contact with the annular surface 154 tofix the bone anchor assembly 400 relative to the bone plate 100, as willbe discussed in greater detail below. Further, the driving end 402 iscomplimentary to the annular surface 154 so that the driving end 402 maybe in contact with the annular surface 154 throughout the range ofinsertion angles of the bone anchor assembly 400.

The upper chamfer 152 tapers radially outward from the annular surface154 to accommodate a bone anchor assembly 400 as it passes into therespective bone plate bore at an angle relative to the bone plate 100.Similarly, the lower chamfer 156 tapers radially outward from theannular surface 154 to accommodate the bone anchor assembly 400 as itextends from the lower surface 112 of the bone plate 100. At thejunction of the annular surface 154 and the upper chamfer 152 there isan upper lip 158 that projects radially inward which restricts back outof the bone anchor assembly 400 from within the bores 102, 104, 106, and108. There is also a lower lip 160 at the junction of the annularsurface 154 and the lower chamfer 156 that generally defines a lowerdiameter that is smaller than at least a portion of the bone anchorassembly driving end 402 such that the lower lip 160 restricts thedriving end 402 from passing completely beyond the bone plate 100.

The bone anchor assembly 400 is generally elongate and includes adriving end 402 and a shank end 404, as shown in FIG. 8. The shank end404 may be threaded or non-threaded according to the desired boneengagement. Alternatively, the shank end 404 may be configured to engagea member (not shown) pre-positioned within the subject bore. In apreferred form, the shank end 404 is a threaded self-tapping designhaving a primary diameter of 6 mm. The driving end 402, on the otherhand, includes a locking cap 500 carried on a head portion 406 of a boneanchor 408, as well as a locking fastener 600 connected to the anchorhead portion 406. In one embodiment, the locking cap 500 includes aplurality of projections 502 a, 502 b, 502 c, and 502 d extendingradially inward from an annular wall 504 of the locking cap 500. Slots506 a, 506 b, 506 c, and 506 d separate the projections 502 and aregenerally aligned with circumferentially spaced depressions 410 a, 410b, 410 c, and 410 d formed in the anchor head portion 406, as shown inFIG. 11. The aligned slots 506 and depressions 410 permit a driver to bepassed axially downward into contact with the depressions 410 to drivethe bone anchor shank end 404 into a bone without turning the lockingcap 500 relative to the anchor head portion 406.

Turning now to FIGS. 9 and 10, the locking cap annular wall 504 extendsaxially along the anchor head portion 406 and includes a radially outerupper end portion 508 that is configured to be shifted radially outwardand into tight engagement with the bore annular surface 154 as thelocking fastener 600 is driven into the anchor head portion 406. In oneembodiment, the radially outer upper end portion 508 may include aplurality of spaced upper circumferentially extending shoulders 512 a,512 b, 512 c, and 512 d that expand apart radially outward toward thebore annular surface 154 as the locking fastener 600 cams against thelocking cap 500, which will be discussed in greater detail below. Theannular wall 504 may also include a lower radially raised arcuate outersurface portion 510 that is complimentary to the curvature of the boreannular surface 154 such that the annular surface 154 urges the outersurface portion 510 radially inward as the bone anchor assembly drivingend 402 is seated within a bone plate through bore.

The locking cap 500 is generally C-shaped with opposing ends 514, 516positioned between projections 502 c, 502 d, as shown in FIG. 10. Thelocking cap 500 is preferably resilient, such that the ends 514, 516 mayexpand apart when the locking cap 500 is installed onto the anchor headportion 406. Moreover, the slots 506 and the gap spacing between theends 514, 516 provide flexibility to the locking cap 500 so that theradially outer upper end portion 508 and projections 502 may expandradially outward.

With the locking cap 500 and locking fastener 600 removed from theanchor head portion 406, an axial bore 412 of the anchor head portion406 is exposed, as shown in FIGS. 11 and 12. The axial bore 412 includesan enlarged diameter upper opening portion 416 that opens to an upperend of the anchor head portion 406 and extends axially downwardtherefrom. The axial bore 412 may have a threaded portion 414. In oneform, the axial bore 412 is partially defined by an inner annularsurface 418 that is uninterrupted and is generally formed by a solidwall portion 420 of the anchor head portion 406. The anchor head portion406 also includes pairs of axially upwardly projecting bosses 422 a, 422b that each have a recess 424 therebetween that is sized and configuredto receive one of the locking cap projections 502. If the locking cap500 begins to rotate relative to the anchor head portion 406, the one ormore projections 406 will rotate into contact with one of thecorresponding bosses 422 a, 422 b. At this point, one of the bosses 422a, 422 b will resist further rotation of the projection 406 to maintainthe locking cap 500 in position on the anchor head portion 406.

As shown in FIG. 12, the anchor head portion 406 also includes anannular channel 430 having a cylindrical inner surface 432 extendingsubstantially parallel to the longitudinal axis of the bone anchor 408.Annular upper and lower surfaces 434 and 436 extend toward thecylindrical inner surface 432 in a direction generally transverse to thebone anchor longitudinal axis. At the upper end of the bone anchor 408,the anchor head portion has a radially outer upper surface 440 thattapers radially inward going toward the upper end and away from theannular channel 430. In one form, the radially outer upper surface 440may be conical.

Drawing attention to FIGS. 13 and 14, additional features of the lockingcap 500 are presented. At the lower end of locking cap 500, a radiallyinner lower end portion 526 is sized and configured to fit into theannular channel 430 of the anchor head portion 406. The lower endportion 526 includes a flat annular upper surface 528 that abuts theannular upper surface 434 of the anchor head portion 406 when thelocking cap 500 is carried on the anchor head portion 406. Additionally,a curved inner surface 530 of the lower end portion 526 contacts and iscomplimentary to the cylindrical inner surface 432 of the anchor headportion 406.

With respect to the interaction between the locking cap 500 and thelocking fastener 600, the locking cap 500 includes a radially innerupper cam surface 520 that cooperates with the locking fastener 600. Inone embodiment, the locking cap projections 502 may have radially innerends 522. Here, the radially inner upper cam surface 520 may be in theform of a plurality of cam surfaces 524 on the radially inner ends 522.In greater detail, the plurality of cam surfaces 524 are arranged to becomplimentary to the locking fastener 600 and are generally inclinedaway from the radially inner ends 522.

As shown in FIGS. 15-17, the locking fastener 600 has an upper headportion 610 that is configured for driving the locking fastener 600 intothe bone anchor bore 412 and is sized to be received within the upperopening portion 416 of the bone anchor bore 412. The locking fastenershank 612 may have a number of different designs to permit the fastener600 to advance longitudinally along the bone anchor bore 412 whileresisting back out, including but not limited to threads, an expandingend, or teeth that engage a pawl. In a preferred embodiment, the lockingfastener shank 612 is threaded to engage the threaded portion 414 of thebone anchor bore 412. The locking fastener shank 612 is adjacent anannular collar 614 of the locking fastener 600. The annular collar 614has a flat upper surface 616, a flat lower surface 618, and a radiallyextending lower cam surface 620 extending therebetween. Axially abovethe flat upper surface 616 is a narrow portion 622 that may be generallycylindrically shaped and leads to a radially extending upper cam surface624. As shown in FIG. 17, the locking fastener upper head portion 610has a driver bore 626 for receiving a hex-lobe driver and an undulatingwall 628 that contacts the lobes of the driver and transfers rotation ofthe tool to the locking fastener 600.

Referring now to FIGS. 18-24, a method of assembling the bone anchorassembly 400 into a preassembled condition will be disclosed. Once thepreassembled condition is complete, the bone anchor assembly 400 isready to be driven through a bone plate through bore and into a bone.Preferably, the assembly process is performed by the manufacturer tolimit preparatory work required for surgery.

The process begins with passing an open end 540 of the locking cap 500over the anchor head portion 406 in direction A once the locking cap 500and bone anchor 408 have been aligned. In the embodiment shown, thelocking cap open end 540 includes the radially inner lower end portion526 that is configured to fit within the channel 430 formed in anchorhead portion 406. As is shown in FIG. 18, the radially inner lower endportion 526 has a lower chamfered portion 542 that is designed to guidethe radially outer upper surface 440 into the locking cap open end 540and thereby expand the open end 540 over the anchor head portion 406.

As the locking cap 500 is advanced in direction A onto anchor headportion 406, the locking cap curved inner surface 530 acts as a lowercam portion against the radially outer upper surface 440 such that theradially inner lower end portion 526 is shifted radially outward indirection B, as shown in FIGS. 19 and 20. Further, an enlarged radiallyouter upper surface 450 of the anchor head portion 406 continues toexpand the radially inner lower end portion 526 as the locking cap 500is advanced in direction A. Eventually, the radially inner lower endportion 526 reaches the bone anchor annular channel 430 and contractsradially into the channel 430 to form a snap-fit connection between thelocking cap 500 and the anchor head portion 406, as shown in FIG. 21.Stated differently, snap-fitting the radially inner lower end portion526 into channel 530 engages the locking cap flat annular upper surface528 and curved inner surface 530 respectively against the anchor headportion annular upper surface 434 and cylindrical inner surface 432 toretain the locking cap 500 on the anchor head portion 406. As theradially inner lower end portion 526 reaches the channel 430, thelocking cap projections 502, if present, should be aligned between thepairs of axially upwardly projecting bosses 422 a, 422 b so that theprojections 502 may be seated therein once the lower end portion 526reaches the channel 430 and the locking cap 500 is seated on the anchorhead portion 406.

FIGS. 21-24 illustrate the step of connecting the locking fastener 600to the anchor head portion 406. The locking fastener shank 612 is passedinto the anchor head portion bore 412 in direction A and into engagementwith bore threaded portion 414 or other surfaces therein. Preferably,the locking fastener shank 612 and the flat lower surface 618 are sizedto permit these features of the locking fastener 600 to pass beyond theradially inner upper cam surface 520 of the locking cap 500 in directionA. However, the radially extending lower cam surface 620 of the lockingfastener is preferably sized to contact the radially inner upper camsurface 520 and cam the surface 520 radially outward in direction B suchthat the larger flat upper surface 616 may pass beyond the radiallyinner upper cam surface 520 and into an intermediate axial positionalong the longitudinal axis of the anchor head portion 406. Preferably,the radially inner upper cam surface 520 will resiliently contractradially inward so that the locking fastener annular collar 614 ispositioned axially below the locking cap radially inner upper camsurface 520, as shown in FIG. 24. In this intermediate axial position,the locking cap 500 may resist back out of the locking fastener 600 indirection C. By positioning the locking fastener annular collar 614below the radially inner upper cam surface 520, the bone anchor 408,locking cap 500, and locking fastener 600 are maintained in thepreassembled condition, as shown in FIG. 24. In the preassembledcondition, the bone anchor assembly 400 is ready to be driven through abone plate bore and into a bone.

In one embodiment, the flat upper and lower surfaces 616, 618 have agenerally circular outer periphery that defines a diameter for each.Further, the distance between radial inner ends 522 is greater than thediameter of the flat lower surface 618 but greater than the diameter ofthe flat upper surface 616 so that the fastener shank 612 and flat lowersurface 618 may pass beyond the plurality of cam surfaces 524 withoutradially expanding the projections 502. As the locking fastener 600continues to advance in direction A, the radially extending lower camsurface 620 will cam against the cam surfaces 524 and expand theprojections 502 until the locking fastener inner collar 614 passesbeyond the projections 502, as shown in FIGS. 22-24. Then, the resilientproperties of the locking cap 500 will tend to shift the projections 502radially inward such that the locking fastener inner collar 614 ispositioned below the locking cap projections 502. Further, the lockingcap projections 502 may include substantially flat lower surfaces 550that extend over the anchor head portion 406. With the locking fastener600 in the intermediate axial position, the flat upper surface 616 isarranged axially below and in overlapping confronting relation with thesubstantially flat lower surfaces 550 such that the flat lower surfaces550 contact and resist the locking fastener 500 if the fastener 500travels in direction C.

An exemplary method of installing the bone plate system 10 onto one ormore bones will now be disclosed in conjunction with FIGS. 25-31. Thelower face 112 of bone plate 100 is placed against the one or more bonesthat are to be rigidly secured by the bone plate 100. A drill or an awl(not pictured) is inserted through bore 108 along axis β, the desiredangle of insertion for the bone anchor assembly 400, to form a hole inthe target bone. The bone anchor assembly 400 is then driven indirection D into the bore 108 until shank end 404 engages the holeformed in the bone. In one embodiment, the shank end 404 is in the formof a screw such that the bone anchor assembly is rotatably driven indirection D until the bone anchor assembly driving head 402 begins toenter the bore 108. As shown, the bone anchor assembly 400 is drivenalong an axis β having an angle δ relative to the normal axis α of thelower surface 112 of the bone plate 110, as taken from the center ofbore 108. Because δ is a relatively large angle, the locking cap annularwall 504 will contact upper lip 158 d as anchor assembly 400 isrotatably engaged with the bone and the bone anchor assembly drivinghead 402 is seated within the bore 108.

As previously discussed, bone anchor assembly 400 may include lockingcap projections 502 extending from the locking cap annular wall 504 withradially inner ends 522. In the preassembled condition, such anembodiment preferably has the locking cap projections 502 aligned withthe locking fastener narrow portion 622 with the locking fastener at theintermediate axial position in the bone anchor 408. The radially innerends 522 are each separated from the locking fastener narrow portion 622by a gap spacing 640. The gap spacing 640 permits one or more radiallyinner ends 522 to shift radially inward due to contact of the lockingcap annular wall portion 504 with the upper lip 158 d or other featuresof the bone plate 100 as the locking cap 500 and anchor head portion 406enter the bore 108. The radially inward shift is best shown by thedifference between FIGS. 25 and 27. In one form, the locking cap 500 mayhave a radially inner upper surface 560 that is separated by a gapspacing 460 from the radially outer upper surface 440 of the anchor headportion 406, as shown in FIG. 26. The gap spacing 460 permits thelocking cap annular wall portion 504 to deflect radially inward as theannular wall portion 504 contacts the upper lip 158 d or other featuresof the bone plate 100.

To ease insertion into the bore 108, the locking cap annular wallportion 504 is preferably arcuate such that the annular wall portion 504will cam against the upper lip 158 d. In this manner, the annular wallportion 504 may be shifted radially inward in direction E as the boneanchor assembly is driven in direction D, as shown in FIG. 28.Additionally, the radially outer upper end portion 508 may have an outercurvature that is flush with the rest of annular wall portion 504 sothat the locking cap 500 will tend not to get caught on upper lip 158 dor other features of the bone plate 100.

Once the anchor head portion 406 and locking cap 500 carried thereon arepositioned within the bore 108, the locking cap 500 is seated againstthe bore annular surface 154 d. In a preferred form, at least thelocking cap radially outer upper end portion 508 is seated against thebore annular surface 154 d due to the smaller diameter of lower lip 160that restricts the bone anchor driving end 402 from fully passing beyondthe bone plate 100. Next, the locking fastener 600 may be driven axiallyinto the anchor head portion 406 to expand the locking cap 500 and fixthe bone anchor assembly 400 relative to the bone plate 100. In theillustrated embodiment, the locking fastener 600 has a threaded shank612 so the locking fastener 600 is rotated in direction F to drive thelocking fastener 600 into the anchor head portion 406.

By driving the bone anchor assembly 400 into the bore 108, the lockingfastener upper cam surface 624 cooperates with the locking cap radiallyinner upper cam surface 520 to deflect the locking cap annular wallportion 504, as shown in FIGS. 29 and 30. Specifically, the locking capradially outer upper end portion 508 shifts radially outward indirection R into tight engagement with the bore annular surface 154 d.Further, the shifting of radially outer upper end portion 508 radiallyoutward pivots the annular wall portion so that the radially inner lowerend portion 528 is shifted inward into tight engagement with the anchorhead portion 406. With respect to embodiments with projections 502 andcam surfaces 524 at radially inner ends 522, driving the lockingfastener 600 into the anchor head portion 406 brings the lockingfastener upper cam surface 624 into contact with the cam surfaces 524such that the projections 502 shift radially outward and expand thelocking cap radially outer upper end portion 508 into tight engagementwith the bore annular surface 154 d.

As the locking fastener 600 is driven deeper into the bone anchor 408,the locking fastener upper cam surface 624 gradually increases in size.Thus, the locking cap radially outer upper end portion 508 tends tocontinue to expand radially outward. However, because the bone plate 100and bore annular surface 154 d are relatively rigid, the bore annularsurface 154 d exerts a reactive force which resists further expansion ofthe locking cap 500 by way of friction against the locking fastenerupper cam surface 624. Accordingly, the firmness of the engagement ofthe locking cap radially outer upper end portion 508 against the boreannular surface 154 d may be controlled by way of the torque applied todrive locking fastener 600 into the bone anchor 408. Once apredetermined amount of torque will no longer rotate the lockingfastener 600, the bone anchor assembly 400 will be fixed relative to thebone plate 100 with a correlated amount of firmness. In this manner, thelocking fastener 600 generally sits proud within axial bore 412 sincethe locking cap radially inner upper cam surface 520 frictionallyresists further rotation of fastener 600 into the axial bore 412. FIG.31 illustrates the bone anchor assembly 400 rigidly secured at angle δwithin bore 108 after the locking fastener 600 has been driven into thebone anchor 408 a predetermined distance to provide the desiredengagement of the locking cap radially outer upper end portion 508against the annular surface 154 d.

Two additional bone plate system embodiments 1010, 2010 are shown inFIGS. 32-42. Bone plate system embodiments 1010, 2010 may include boneanchor assembly 400 or an alternative embodiment, bone anchor assembly1400. Bone plate system embodiments 1010, 2010 are similar to the boneplate system 10 in that preassembled bone anchor assemblies 400, 1400may be polyaxially driven through bores in the bone plates 1100, 2100and fixed at an angle relative to the bone plates 1100, 2100.Additionally, the bone anchor assemblies 400, 1400 have lockingfasteners 600, 1600 that are driven into bone anchor head portions 406,1406 to expand locking caps 500, 1500 carried thereon.

Bone plate system 1010 has an elongated bone plate 1100 with a dog-boneshape similar to the bone plate 100, as shown in FIG. 32. The bone plate1100 has upper and lower surfaces 1110 and 1112 and a sidewall 1116extending about the periphery of the bone plate 1100. The bone plate1100 generally has a narrow middle portion 1118 and wider opposing ends1120, 1122. Preferably, the middle portion 1118 has a width of 19 mm andthe opposing ends 1120, 1122 have a width of 26 mm. Further, the boneplate 1100 preferably has a thickness of 3.5 mm at the middle portion1118 and a thickness of 5.0 mm at the opposing ends 1120, 1122. Thesidewall 1116 may include a generally flat outer surface 1134 and upperand lower curved outer surfaces 1136, 1138 that form a smooth transitionbetween the generally flat outer surface 1134 and the bone plate upperand lower surfaces 1110, 1112. To aid in installation, the bone plate1100 includes installation tool bores 1200 a, 1200 b that receive theexpanding ends of an installation tool (not shown). Flat annularsurfaces 1202 a, 1202 b are provided about the bores 1200 a, 1200 b topermit the installation tool to firmly abut the flat annular surfaces1202 a, 1202 b and maintain a secure connection between the tool and thebone plate 1100.

The bone plate 1100 also has temporary holding pin bores 1130, 1132positioned at the outermost ends 1120, 1122 of the bone plate 1100. Ifthe bone plate 1100 is placed on two adjacent vertebral bodies, theseparation of the holding pin bores 1130, 1132 permits the holding pinsto be placed away from the end plates of the vertebral bodies, whichwould be positioned toward the middle of the bone plate 1100. Further,using bore 1130, 1132 limits movement of the bone plate 1100 away fromthe temporary holding pins even if there is only one pin used totemporarily maintain the bone plate 1100 against the bone.

Positioned at the corners of the bone plate 1100 are through bores 1102,1104, 1106, and 1108, as shown in FIG. 33. The bone plate 1100 includesbore sidewalls 1150 a, 1150 b, 1150 c, and 1150 d that generally definea respective bore. Like the sidewalls 150 of bone plate 100, each boresidewall 1150 includes an upper chamfer 1152, an upper lip 1158, a boreannular surface 1154, a lower lip 1160, and a lower chamfer 1156, asshown in FIGS. 34 and 35. The upper chamfer 1152 provides clearance fora bone anchor assembly 1400 as it is inserted into, for example, bore1108. The upper chamfer 1152 generally tapers radially inward to upperlip 1158 d, which has a smaller diameter to resist back out of the boneanchor assembly 1400. The bore annular surface 1154 d forms a generallyspherical pocket that the locking cap 1500 expands into contact with tofix the bone anchor assembly 1400 within the bore 1108. The lower lip1160 d preferably has a smaller diameter than the upper lip 1158 d sothat the lower lip 1160 d restricts the bone anchor assembly 1400 frompassing completely therethrough.

In some instances, the procedure of driving the bone anchor assembly1400 and the contact of the locking cap 1500 against the bone plate 1100may damage the bones sought to be fused. Here, it may be desirable toreduce the interference between the upper lip 1158 and the locking cap1500. A preferred embodiment that addresses this issue is shown in FIG.36. Specifically, the upper lip 1158 includes a flat annular wall 1157 dpositioned above the bore annular surface 1154 d. In this embodiment,the upper lip 1158 d effectively has a larger opening so that the boneanchor assembly 1400 may pass through with less interference between thelocking cap 1500 and the bone plate 1100. One downside to thisconfiguration is that the increased size of upper lip 1158 d decreasesthe amount lip 1158 d projects over the locking cap 1500 which reducesthe ability of the lip 1158 d to resist back-out of bone anchor assembly1400. This configuration of upper lip 1158 may be used in any bone platesystem embodiment.

Turning now to FIG. 37, bone plate system 2010 includes a bone plate2100 that is similar in many respects to bone plate 1100. For example,bone plate 2100 has upper and lower surfaces 2110, 2112 and a sidewall2116 extending around the periphery of the bone plate 2100. Preferably,the sidewall 2116 is curved to limit interference with soft tissues. Thebone plate 2100 also has a dog-bone shape with a narrow middle portion2118 and wider opposing ends 2122. In a preferred embodiment, the middleportion 2118 has a width of 19 mm and the opposing ends have a width of26 mm. To aid in installation, the bone plate 2100 has temporaryinstallation tool bores 2200 a, 2200 b configured to receive theexpanding ends of an installation tool (not shown) and flat annularsurfaces 2202 a, 2202 b to provide a flat abutting surface for theinstallation tool. Additionally, the bone plate 2100 has temporaryholding pin bores 2130, 2132 positioned at the outermost ends 2120, 2122of the bone plate 2100.

One difference between bone plates 2010 and 1010 is the presence of alower projection 2204 on bone plate 2010. As shown in FIGS. 38 and 39,the bone plate 2100 has lower projection 2204 and a seating projection2206 extending from the bone plate lower surface 2112. The seatingprojection 2206 is spaced from the lower projection 2204 such that thebone plate 2100 may be positioned on the L5 vertebra 2012 and the S1vertebra 2014 with the seating projection 2206 resting on a sacralpromontory 2018 and the lower projection 2204 contacting the S1 vertebra2014. With respect to the seating projection 2206, the temporaryinstallation tool bore 2200 a may intersect the seating projection 2206such that the seating projection 2206 is comprised of two aligned,spaced portions extending laterally across the bone plate 2100. Further,the seating projection 2206 may include a flat inclined surface 2208 forcontacting the sacral promontory 2018 and a curved inclined surface 2210that generally faces the L5 vertebra 2012.

The lower projection 2204 extends from the bone plate 2100 at end 2120and is generally curved about the longitudinal axis of the bone plate2100 to conform to bone surfaces, as shown in FIGS. 39-42. The lowerprojection 2204 includes an undercut 2212 inclined away from the boneplate lower surface 2112 that increases in height heading away from theseating projection 2206. An inner wall portion 2220 extends between theundercuts 2212 a, 2212 b and has a curvature that matches bone plate end2120. In a preferred form, the undercut 2212 is dimensioned such thatthe lower projection 2204 contacts the S1 vertebra at points 2020 a,2020 b which are laterally spaced from each other on the lowerprojection 2204, as shown in FIGS. 39 and 41.

The bone plate 2100 includes bores 2102, 2104, 2106, and 2108 that areeach preferably configured to receive bone anchor assemblies 1400 thatare polyaxially driven therethrough. The bone plate 2100 has boresidewalls 2150 that each define a bore and include an upper chamfer2152, an upper lip 2158, a bore annular surface 2154, a lower lip 2160and a lower chamfer 2156. The sizing and configuration of the sidewalls2150 are similar to the embodiments of bone plates 100 and 1100 andprovide similar functionality with respect to receiving and retainingbone anchor assemblies.

As shown in FIG. 43, bone anchor assembly 1400 is another embodiment ofa preassembled bone anchor that may be used in any of the bone platesystem embodiments disclosed herein. Bone anchor assembly 1400 has ashank end 1404 that may have a number of different configurations,including a screw, a relatively smooth shank, or even a threadedconfiguration for engaging a member disposed in bone. In a preferredform, shank end 1404 is a self-tapping bone screw. The bone anchorassembly 1402 has an opposing driving end 1402 that is configured to bedriven into a bone plate through bore.

The bone anchor assembly 1400 includes a bone anchor 1408 having a boneanchor head portion 1406 with a locking cap 1500 mounted thereon. Alocking fastener 1600 is connected to the anchor head portion 1406 whenthe bone anchor assembly 1400 is in a preassembled configuration.Further, the locking fastener 1600 is driven into the anchor headportion 1406 to expand the locking cap 1500 and fix the bone anchorassembly 1400 within a bone plate bore.

Though the locking cap 1500 is similar to the locking cap 500, there areseveral differences between the two embodiments. Generally, the lockingcap 1500 has an annular wall portion 1504 that extends axially along thebone anchor head portion 1406 with the locking cap 1500 carried on theanchor head portion 1406. However, instead of a C-ring design, thelocking cap 1500 includes a plurality of close-ended upper and lowerslots 1506, 1576 axially extending along the locking cap 1500. Theplurality of slots 1506, 1576 permit portions of the locking cap 1500 toflex inwardly and outwardly as required to accept the anchor headportion 1406 and to fix the bone anchor assembly 1400 within a boneplate bore, as will be discussed below. By using a number of lower slots1576, expansion of the locking cap 1500 is more evenly distributedaround the locking cap 1500, thereby minimizing stress concentrationswhich could potentially yield the locking cap 1500 material.Additionally, it has been found that using a number of lower slots 1576increases the retention load of the locking cap 1500 on the anchor headportion 1406. A greater retention load tends to keep the locking cap1500 on the anchor head portion 1406 despite interference with, forexample, upper lip 2158 b as the bone anchor assembly 1400 is driveninto bore 2104.

With continued reference to FIG. 43 and the bone plate system 2010 ofFIG. 42, the locking cap annular wall 1504 includes a lower radiallyraised arcuate outer surface portion 1510 configured for engaging thebone plate annular surface 2154. Preferably, the outer surface portion1510 includes a plurality of outer surfaces 1511 that are separated bythe lower slots 1576 and are complimentary to the bone plate boreannular surface 2154. This way, the outer surface portion 1510 tends toshift the locking cap annular wall 1504 radially inward as the boneanchor assembly driving end 1402 is seated within a bone plate bore.Also, above the lower arcuate outer surface portion 1510 is a radiallyrecessed outer surface portion 1570.

At the other end of annular wall 1504 is a radially outer upper endportion 1508 that is expanded into contact with the bore annular wall2154. The outer upper end portion 1508 includes an upper radiallyraised, arcuate outer surface portion 1572 configured to engage the boreannular surface 2154. Thus, the upper and lower radially raised arcuateouter surface portions 1572, 1510 are positioned at opposite ends of thelocking cap 1500 and are separated by a radially recessed outer surfaceportion 1570. In this manner, there is a spaced contact between theraised arcuate outer surface portions 1572, 1510 and the bore annularsurface 2154.

As shown in FIG. 44, the bone anchor head portion 1406 includes aplurality of depressions 1410 that are aligned with upper slots 1506 inthe locking cap 1500 when the lock cap 1500 is carried on the anchorhead portion 1406. In a manner similar to bone anchor assembly 400, adriver may be inserted into contact with the depressions 1410 to drivethe bone anchor assembly 1400 through a bone plate bore and intoengagement with a bone without rotating the locking cap 1500 relative tothe anchor head portion 1406. In one embodiment, the locking cap 1500also has projections 1502 that extend radially inward from the lockingcap annular wall 1504. Further, the anchor head portion 1406 may haveaxially upwardly projecting bosses 1422 a, 1422 b that receive one ofthe projections 1502 therebetween to resist rotation of the locking cap1500. The locking cap 1500 may also have a plurality ofcircumferentially extending shoulders 1574 spaced by the upper slots1506 that are expanded into contact with the bore annular surface 2154and may contact the bore upper lip 2158 to resist back out of boneanchor 1408.

The bone anchor head portion 1406 has a central axial bore 1412 incommunication with an upper opening portion 1416 at the upper end of thebone anchor 1408. A solid wall portion 1420 of the anchor head portion1406 may extend axially with an inner annular surface 1418 thereofdefining a portion of the axial bore 1412. The axial bore 1412 mayinclude a threaded portion 1414 for receiving a threaded shank of thelocking fastener 1600. The bone anchor head portion 1406 also has aplurality of circumferentially spaced depressions 1410. As best shown inFIG. 45, the pairs of bosses, 1422 a, 1422 b are positioned on eitherside of a recess 1424 that is sized to receive one of the locking capprojections 1520.

To connect the locking cap 1500 to the bone anchor 1408, the anchor headportion 1406 includes a radially outer upper surface 1440 that becomesprogressively wider approaching an annular channel 1430. The radiallyouter upper surface 1440 expands the locking cap 1500 over the anchorhead portion 1406 before a radially inner lower end portion 1526 of thelocking cap 1500 snaps into the annular channel 1430. In one embodiment,the radially outer upper surface 1440 may be conical. The annularchannel 1430 includes a cylindrical inner surface 1432 and upper annularsurface 1436 that are configured to restrict longitudinal movement ofthe locking cap 1500 along the bone anchor 1408.

With respect to the locking cap 1500, the projections 1502 includefillets 1580 a, 1580 b on either side thereof that reduce the width ofthe projections 1502, as shown in FIGS. 47 and 48. Though narrower, theprojections 1502 are sufficiently strong to shift and hold the lockingcap radially outer upper end portion 1508 in tight engagement with, forexample, the bore annular surface 2154. The narrower projections 1502also permit the bosses 1422 a, 1422 b to be larger without reducing thesize of spaced depressions 1410 so that the bosses 1422 a, 1422 b aremore rigid and resist shearing forces caused by rotation of the lockingcap 1500.

The locking cap 1500 also has a radially inner upper cam surface 1520that cooperates with the locking fastener 1600 to deflect the lockingcap radially outer upper end portion 1508 radially outwardly as thelocking fastener 1600 is driven into the bone anchor bore 1412. In theembodiment shown, the locking cap 1500 has projections 1502 with radialinner ends 1522 and a plurality of cam surfaces 1524 formed on theradial inner ends 1522. Here, the plurality of cam surfaces 1524cooperate with the locking fastener 1600 to shift the projections 1502radially outward.

As the radially outer upper end portion 1508 expands, the locking capannular wall 1404 tends to pivot such that the radially inner lower endportion 1526 is brought into tight engagement with the bone anchor headportion 1406. In one embodiment, the lower end portion 1526 includes aplurality of circumferentially extending ribs 1530 spaced by lower slots1576. Due to the radially inward movement of the lower end portion 1526,the ribs 1530 contract together within the bone anchor annular channel1430 to tightly engage the anchor head portion 1406.

The locking fastener 1600 is similar to the locking fastener 600, as isapparent from FIGS. 49-51. The locking fastener 1600 includes a fastenershank 1612 that may be threaded, an annular collar 1614, and an enlargedupper head portion 1610. A narrow portion 1622 is positioned between thecollar 1614 and the head portion 1610 and is aligned with the lockingcap radial inner ends 1522 when the locking fastener 1600 is connectedto the bone anchor member 1406 at an intermediate axial position. Thereis preferably a gap spacing between the narrow portion 1622 and theradially inner ends 1522 to permit the projections 1502 to shiftradially inward due to contact of the locking cap annular wall 1504 withthe bone plate 100, 1100, or 2100 during insertion of the bone anchorassembly 1400 into a bone plate through bore.

Another similarity is that the locking fastener annular collar 1614includes a flat upper surface 1616 that is in confronting overlappingrelation with a lower surface of the projections 1502 when the lockingfastener 1600 is in the intermediate axial position. To reach theintermediate axial position, the annular collar radially extending lowercam surface 1620 cooperates with the locking cap radially inner uppercam surface 1520 to shift the upper cam surface 1520 or in someembodiments, shifts the projections 1502, out of the path of the lockingfastener. Additionally, the annular collar 1614 has a radially extendingupper cam surface 1620 that cooperates with the locking cap radiallyinner upper cam surface 1520 to expand the locking cap radially outerupper end portion 1508 into engagement with the bore annular surface154, 1154, or 2154.

One difference between the locking fasteners 600, 1600 is that lockingfastener 1600 has a driver bore 1626 for receiving a three-lobed bore.This is the preferred approach, as using a higher number of lobesrequires smaller corresponding features within the bore that may shearoff during rotation.

Given the similar designs of bone anchor assemblies 400, 1400, themethods of assembling bone anchor assembly 1400 into the preassembledconfiguration wherein the locking cap 1500 and locking fastener 1600 areconnected to the bone anchor 1408 are substantially similar. Likewise,the process of driving the bone anchor assembly 1400 into a through boreof bone plates 10, 1010 or 2010 and fixing the bone anchor assembly 1400therein is similar to the process disclosed with respect to bone anchorassembly 400.

However, in one embodiment, the upper radially raised, arcuate outersurface portion 1572 of the locking cap 1500 includes a lower camsurface 1584, as shown in FIGS. 52 and 53. As the bone anchor assembly1400 is driven into bone plate bore 1108, the lower cam surface 1584cooperates with the upper lip 1158 d and shifts the radially outer upperend portion 1508 radially inward. Once the locking cap upper end portion1508 passes the upper lip 1158 d, the upper end portion 1508 shiftsradially outward before being fixed into position by set screw 1600.

The components of bone plate system embodiments 10, 1010, 2010 arepreferably made from titanium alloy. The locking caps 500, 1500 arepreferably resilient to deflect and expand as required, yet strongenough to fixedly maintain the bone anchor assemblies 400, 1400 withinthe bone plates 100, 1100, 2100. It is appreciated that a number ofother materials may be acceptable for the different components of thebone plate system embodiments. These materials preferably have a highstrength-to-weight ratio, are biocompatible, and are sufficientlydurable to potentially remain within the patient for an extended periodof time. Metallic alloys and polymers including polyetheretherketone(PEEK) are also contemplated for use as one or more components of thebone plate system embodiments.

In FIG. 54, another embodiment of a bone plate system 3010 is presented.Bone plate system 3010 includes a bone plate 3100 and a plurality ofbone anchor assemblies 3400 received within through bores 3102 of thebone plate 3100. The bone plate system 3010 has a relatively shortprofile and in one embodiment is configured to partially extend into theintervertebral space between vertebra. Further, the bone plate system3010 preferably includes a general V-shape which positions the boneanchor assemblies 3400 at an angle relative to the vertebrae so that thebone anchor assemblies 3400 may be driven diagonally into the corner ofeach vertebra.

Each bone anchor assembly 3400 includes a bone anchor 3402, a lockingcap 3500 carried on the bone anchor 3402, and a locking fastener 3600engaged with a bone anchor head portion 3404. Preferably, the boneanchor assembly 3400 is preassembled and ready to be driven through abone plate through bore 3102 and into a bone. Once the bone anchor headportion 3404 is received within the bore 3102, the locking fastener 3600is driven axially further into the bone anchor head portion 3404 toexpand the locking cap 3500 and resist back out of the bone anchorassembly 3400 from the bore 3102.

More specifically, the bone plate 3100 has a relatively thick middleportion 3104 between opposing ends 3106 a, 3106 b. At the ends 3106 a,3106 b there are corresponding temporary pin notches 3108 a, 3108 b.Also near the ends are transition portions 3110 a, 3110 b thattransition between the relatively thick middle portion 3104 to thethinner ends 3106. Bone plate 3100 also has a tool insertion bore 3112where an insertion tool with an expandable end is preferably connectedto position the bone plate 3100 on bone.

As shown in FIG. 55, the bone plate 3100 may be positioned on ananterior vertebral surface 3016 between two vertebrae 3012 such that themiddle portion 3104 projects partially into the intervertebral space3014. The bone plate 3100 includes an upper surface 3116 that faces awayfrom the vertebrae 3012 and lips 3118 a, 3118 b at the ends 3106 a, 3106b that are thin and extend over the anterior vertebral surface 3016. Ina preferred embodiment, the bone plate 3100 has a sidewall 3120 withundercuts 3122 a, 3122 b that are configured to conform to the curvatureof the adjacent vertebrae 3012.

The bone plate 3100 may also be generally curved to conform to theanterior surfaces 3016 or other bone features, as shown in FIG. 56. Toreceive the end of a bone, the bone plate 3100 may have arcuate surfaces3130 a, 3130 b which may seat against a corner of the bone. FIG. 57presents a bottom view of the bone plate 3100 that shows the arcuatesurfaces 3130 a, 3130 b meeting at a lower surface 3132. Also present onthe bone plate 3100 are raised ridges 3134 a, 3134 b that project awayfrom the respective arcuate surface 3130 a, 3130 b and may be configuredto engage bone surfaces.

The bone plate 3100 preferably includes four bores 3102 a, 3102 b, 3102c, and 3102 d generally defined by respective bore sidewalls 3140 a,3140 b, 3140 c, and 3140 d, as shown in FIGS. 58-60. Each bore sidewall3140 includes an upper bore surface 3142, a lower bore surface 3144, anannular seating surface 3146, and an arcuate channel 3148. The lowerbore surface 3144 has the smallest opening, which restricts the anchorhead portion 3404 from passing completely through the bore 3102. Theanchor head portion 3404 rests against annular seating surface 3146, sothe annular seating surface 3146 is preferably sized larger than thelower bore surface 3144. The upper bore surface 3142 a is smaller thanthe arcuate channel 3148 to restrict back out of the bone anchorassembly 3400 once the locking cap 3500 has expanded within the arcuatechannel 3148 to a size larger than the opening defined by the upper boresurface 3142.

The bone anchor assembly 3400 includes a head portion 3404 and a shankportion 3405. The head portion includes a cylindrical outer surface 3406that is sized to be in tight engagement with lower bore surface 3144when placed into a through bore 3102. The head portion 3404 alsoincludes projections 3408 that serve to retain the locking cap 3500 onan upper radially extending portion 3410. The shank portion 3405 may bethreaded or non threaded, but is preferred to be of a self-tapping bonescrew configuration.

With reference to FIGS. 62-64, the locking cap 3500 includes an outerring 3502 and projections 3504 a, 3504 b that extend radially inwardfrom the outer ring 3502. The projections 3504 a, 3504 b have radiallyinner ends 3056 a, 3506 b and cam surfaces 3508 a, 3508 b disposedthereon. The outer ring 3502 is generally C-shaped with a gap spacing3510 between ends thereof.

The bone anchor head portion 3404 carries the locking cap 3500 on theupper radially extending portion 3410 that extends about the peripheryof the anchor head portion 3404. As shown in FIG. 63, the anchor headportion 3404 may include four projections 3408 a, 3408 b, 3408 c, and3408 d extending axially upward from the upper radially extendingportion 3410. To position the locking cap 3500 onto the anchor headportion 3404, the locking cap gap spacing 3510 is expanded and thelocking cap 3500 is translated toward the anchor head portion 3404 andalong the longitudinal axis of the bone anchor 3402 until the lockingcap projections 3504 a, 3504 b are received within channels 3412 formedbetween projections 3408. Then, the locking cap 3500 is released suchthat the outer ring 3502 resiliently snaps into undercuts 3414 formed inthe projections 3408. Also shown in FIG. 63 is an axially extending bore3420 of the bone anchor 3402. The axially extending bore 3420 is incommunication with an opening 3422 at the upper end of bone anchor 3402.In a preferred form, the bore 3420 has a threaded portion 3424.

Once the locking cap 3500 is positioned on the anchor head portion 3404,the locking fastener shank 3612 is advanced into the axially extendingbore 3420. In a preferred form, the locking fastener shank 3612 isthreaded to engage the threaded portion 3424 of axially extending bore3420. The locking fastener 3600 has an annular collar 3614 that includesflat upper and lower surfaces 316, 318, as well as a radially extendingouter cam surface 3620. As the locking fastener 3600 is axially advancedalong the bone anchor head portion 3402, the locking cap cam surfaces3508 cooperate with the locking fastener outer cam surface 3620 to shiftthe locking cap projections 3504 radially outward which permits theannular collar 3614 to pass by the projections 3504. With the lockingfastener annular collar 3614 axially below the projections 3504, thelocking fastener flat upper surface 3616 is in overlapping andconfronting relation with a lower surface 3520 of the projections 3504.The bone anchor assembly 3400 is now assembled in such a manner that thelocking cap 3500 and locking fastener 3600 are retained on the anchorhead portion 3404. The bone anchor assembly 3400 is now ready to bedriven into a bone plate bore 3102.

With respect to FIGS. 64 and 65, the bone anchor assembly 3400 is drivenin direction I into bore 3102 d. The bone anchor cylindrical outersurface 3406 is received in close contact with the lower bore surface3144 d such that the lower bore surface 3144 d to securely hold the boneanchor assembly 3400 within the bore 3102 d. As shown, the bone anchorradially extending portion 3410 is seated against the bone plate annularseating surface 3146 d. Further, the locking cap 3500 is substantiallyaligned with the arcuate channel 3148 d. The bone anchor 3400 may befixed within the bone plate 3100 by driving the locking fastener 3600 indirection I into the axially extending bore 3420.

In the preassembled condition, the radially inner ends 3508 aresubstantially aligned with a narrow portion 3622 of the locking fastenersuch that the projections 3504 may translate radially inward as needed.Driving the locking fastener 3600 brings a radially extending upper camsurface 3624 of the locking fastener 3600 into contact with cam surfaces3508 disposed on the projections 3504. This contact shifts theprojections 3504 and the outer ring 3502 radially outward. The lockingfastener 3600 continues to translate in direction I until projections3504 pass shoulder 3626 and arrive into contact with an enlarged annularsurface 3628. Here, the projections 3504 are forced to remain in theirradially extended positions which maintain the locking cap 3500 in theexpanded state which resists back out of the bone screw assembly 3400.Additionally, an upper lip 3630 may restrict further movement of thelocking fastener 3600 in direction I. FIG. 66 shows the bone platesystem 3010 with the bone anchor 3400 fixed within the bone plate 3100.

Several tools associated with the bone plate system embodimentsdiscussed above will now be disclosed. It is contemplated that the toolsmay be configured to be used in any of the bone plate system embodiments10, 1010, 2010, and 3010, but features of particular bone plate systemembodiments will be used for exemplary purposes. One approach toinserting the bone anchor assembly 1400 involves using a bone anchordriver 4000, as shown in FIG. 67. The bone screw driver has a handle end4002 and a driver end 4004. The handle end 4002 is preferably a quarterinch square drive configuration adapted to be received within a modularratcheting T-handle. The driver end 4004 has four prongs 4006 a, 4006 b,4006 c, and 4006 d, which may engage the spaced depressions 1410 on boneanchor head portion 1406. At the base of the prongs 4006 is a flatannular surface 4008 that contacts the locking cap 1500. The driver end4004 also includes a bore 4010 extending axially along the bone anchordriver 4000 that houses a press fit member 4012. The press fit member4012 has a plurality of resilient fingers 4014 that are biased radiallyoutward but deflect radially inward when the press fit member 4012 ispressed into a driver bore 1626. The radial bias of press fit member4012 engages the locking fastener 1600 and the rest of the bone anchorassembly 1400 to the driver end 4004. A user may then drive the boneanchor assembly 1400 through a bone plate bore and into bone.

A final driver 5000 shown in FIG. 68 is used to rotate the lockingfastener 1600 and expand the locking cap 1500 to fix the bone anchorassembly 1400 within a bone plate bore. The final driver 5000 has ahandle end 5002 and a driver end 5004. The handle end 5002 is preferablyconnected to a modular torque limiting T-handle to restrictover-tightening of locking fastener 1600. The driver end 5004 includes atapered portion 5006 that leads to a tri-lobed driver 5008. Alternativedriver configurations, such as a hex or other driver may be used.

To temporarily hold the bone plate system 10, 1010, 2010, 3010 onto thebone, a self-centering threaded pin inserter 6000 may be used. The pininserter 6000 includes a handle 6002, a knob 6004, an inner tool shaft6006, and an outer tool shaft 6008, as shown in FIG. 69. At one end ofthe pin inserter 6000 is a threaded male end 6010 surrounded by aplurality of fingers 6012. The pin inserter 6000 has an internal spring(not shown) that generally maintains the inner tool shaft 6006 and outertool shaft 6008 in an expanded configuration, as shown in FIG. 69.However, the inner tool shaft 6006 may be shifted into outer tool shaft6008 in direction X.

More specifically, the threaded male end 6010 is configured to engage athreaded bore 6108 of a threaded pin 6100, as shown in FIG. 70. The pininserter fingers 6012 are also sized to fit within depressions 6106formed in anchor head portion 6102. In an initial preassembled state,the threaded pin 6100 is held within a bore 6204 of a pin cap 6200 byresilient fingers 6202. The pin cap 6200 is received within an opening6302 formed in a pin centering tip 6300. In the preassembled state, thethreaded pin 6100, pin cap 6200, and pin centering tip 6300 are rigidlyheld together. In an alternative embodiment, the pin cap 6200 and thepin centering tip 6300 may be integrally formed.

As an example, once the bone plate 1100 is in position against a targetbone, the pin inserter tool may be used to drive the threaded pin 6100into the bone to temporarily hold the bone plate 1100 in place.Initially, the threaded male end 6010 is inserted into the head portion6102 of the threaded pin 6100. Holding the handle 6002 firmly, the knob6004 is turned clockwise to rotate the threaded male end 6010 and engagethe threaded male end 6010 to the pin threaded bore 6108. Once the knob6004 is fully tightened, the threaded pin 6100, pin cap 6200, and pincentering tip 6300 are firmly attached to the pin inserter 6000.

Next, an arcuate collar 6306 of the pin centering tip 6300 is insertedinto bone plate bore 1108 and seated against the bore annular surface1154 d. Preferably, the arcuate collar 6306 is complimentary to theannular surface 1154 d such that the arcuate collar 6306 firmly seatstherein and is centered within the bore 1108. The pin centering tip 6300includes slits 6304 formed therein which generally divide the arcuatecollar 6306 into portions that may flex relative to one another. Inconjunction with a generally curved outer profile of the arcuate collar6306, this flexibility permits the pin insertion tool 6000 to beadjusted to the desired bone anchor insertion angle.

Once the pin insertion tool 6000 is at the desired bone anchor insertionangle, the handle is shifted in the X direction. As shown in FIG. 71,this movement releases the threaded pin 6100 from the pin cap 6200 anddrives the threaded pin 6100 in direction Y through the interior of pincentering tip 6300 and into contact with the underlying bone. To drivethe threaded pin 100 to the bone, the user continues to press handle6002 in direction X and rotates handle 6002 clockwise until the threadedpin head portion 6102 is seated against a narrow portion 6310 of the pincentering tip 6300, as shown in FIG. X. Since the arcuate collar 6306 iscentered within the bore 1108 and includes the narrow portion 6310 whichdefines a central opening 6312 for the threaded pin 6100, the threadedpin 6100 is effectively self-centered within the bone plate bore 1108.

At this point, the threaded pin 6100 has securely pinned the bone plate1100 against the bone. The force on handle 6002 in direction X isgradually released so that the inner spring may shift the inner toolshaft 6006 back to its initial condition. Knob 6004 is then rotated todisengage the threaded male end 6010 from the threaded bore 6108. Onceat least one bone anchor assembly 1400 has been driven into anotherbore, the threaded pin 6100 may be removed by reversing the procedure.The threaded pin 6100 preferably has a major diameter that is smallerthan the major diameter of the bone anchor assembly 1400 so that a boneanchor assembly 1400 may be driven into a bone hole formed by thethreaded pin 6100. Further, the threaded pin 6100 preferably has a minordiameter that eliminates the need to re-awl the bone hole beforeinserting the bone anchor assembly 1400.

While there have been illustrated and described particular embodimentsof the present invention, it will be appreciated that numerous changesand modifications will occur to those skilled in the art, and it isintended in the appended claims to cover all those changes andmodifications which fall within the true spirit and scope of the presentinvention.

1. A bone plate system for being fastened to one or more bones, the boneplate system comprising: a bone plate; at least one through bore of theplate having an annular surface extending thereabout; an elongate boneanchor for extending through the bone plate bore and driving into abone, the bone anchor having a longitudinal axis extending between upperand lower ends thereof; a rigid upper head portion of the bone anchorhaving a central axial bore including an enlarged diameter upper openingportion thereof that opens to the bone anchor upper end and extendsaxially downward therefrom; a locking cap carried on the anchor headportion; a locking fastener configured to be inserted into the boneanchor bore for shifting the locking cap into tight engagement with thebone plate annular surface to keep the anchor fixed relative to the boneplate; an upper head portion of the locking fastener configured fordriving the locking fastener into the axial bore of the bone anchor andbeing sized to be received in the upper opening portion of the boneanchor bore; an annular wall portion of the locking cap that extendsaxially along the bone anchor longitudinal axis and about the boneanchor head portion with the locking cap carried on the anchor headportion; a radially outer upper end portion of the locking cap annularwall portion configured to engage the bore annular surface; a radiallyinner lower end portion of the locking cap annular wall portion forengaging the bone anchor; and a radially inner upper cam surface of thelocking cap that cooperates with the locking fastener so that as thelocking fastener is inserted in the bone anchor bore, the lockingfastener cams the locking cap to deflect the annular wall portion withthe radially outer upper end portion shifting radially outward intotight engagement with the bore annular surface and the radially innerlower end portion shifting radially inward into tight engagement withthe bone anchor.
 2. The bone plate system of claim 1 wherein the lockingcap has a plurality of projections extending radially inward from theannular wall portion over the upper head portion of the bone anchor withthe radially inner upper cam surface being a plurality of cam surfacesformed on radially inner ends of the projections to cooperate with thelocking fastener to radially translate the projections and deflect theannular wall portion as the locking fastener is inserted into the boneanchor bore.
 3. The bone plate system of claim 1 wherein the boneanchor, locking cap, and locking fastener are arranged in a preassembledconfiguration with the locking fastener including an annular collar andthe locking fastener being inserted in the anchor head portion bore at apredetermined axial position so that the annular collar portion ispositioned axially below the locking cap radially inner upper camsurface to resist locking fastener back out and maintain the anchor,locking cap, and locking fastener in the preassembled configuration. 4.The bone plate system of claim 1 wherein the radially inner lower endportion of the locking cap comprises a plurality of spaced lowercircumferentially extending ribs which contract together radially inwardtoward the bone anchor as the locking fastener cams against the lockingcap.
 5. The bone plate system of claim 1 wherein the radially outerupper end portion of the locking cap comprises a plurality of spacedupper circumferentially extending shoulders that expand apart radiallyoutward toward the bore annular surface as the locking fastener camsagainst the locking cap.
 6. The bone plate system of claim 1 wherein theannular wall portion has a lower radially raised arcuate outer surfaceportion configured for engaging the bore annular surface and a radiallyrecessed outer surface portion above the lower arcuate outer surfaceportion.
 7. The bone plate system of claim 6 wherein the annular wallportion has an upper radially raised, arcuate outer surface portion atthe radially outer upper end portion thereof configured for engaging thethrough bore annular surface so that the upper raised outer surfaceportion and the lower raised outer surface portion are in spaced contactwith the through bore annular surface with the recessed outer surfaceportion spaced from the through bore annular surface.
 8. The bone plateof claim 1 wherein the bone anchor rigid upper head portion has anannular channel that extends thereabout below the upper end of the boneanchor and the locking cap radially inner lower end portion is sized andconfigured to fit in the channel such that the radially inner lower endportion shifts radially inward into the channel as the locking fastenercams the locking cap.
 9. A bone plate system for being fastened to oneor more bones, the bone plate system comprising: a bone plate having atleast one through bore and a central axis extending therethrough; anelongate bone anchor having a longitudinal axis and being configured forpolyaxial insertion through the bone plate bore; an upper head portionof the bone anchor configured to be received within the bore; a lockingcap carried on the anchor head portion; an annular wall portion of thelocking cap that extends axially along the bone anchor longitudinal axisand about the bone anchor upper head portion with the locking capcarried on the anchor head portion; a plurality of projections of thelocking cap extending radially inward from the annular wall portion; aset screw threadingly engaged to the anchor head portion at apredetermined axial position relative thereto and having a narrowportion thereof that is aligned with the projections of the locking capwith the set screw threadingly engaged to the anchor head portion at thepredetermined axial position; and radially inner ends of the projectionsspaced from the narrow portion of the set screw to permit theprojections to shift radially inward due to contact of the locking capannular wall portion with the bone plate during insertion of the boneanchor through the through bore at large insertion angles between thebone anchor longitudinal axis and the bore central axis.
 10. The boneplate system of claim 9 wherein the anchor head portion has a radiallyouter upper surface and the annular wall portion of the locking cap hasa radially inner upper surface separated from the anchor head portionradially outer upper surface by a gap spacing for allowing the lockingcap annular wall portion to deflect radially inward as the annular wallportion contacts the bone plate during insertion of the bone anchorthrough the bone plate through bore at large insertion angles.
 11. Thebone plate system of claim 9 wherein the set screw has a cam surfaceabove the locking cap projections with the cam surface tapering radiallyoutward from the set screw narrow portion for camming against theradially inner ends of the projections to shift the projections radiallyoutward and the locking cap into tight engagement with the bone plate asthe set screw is driven into the anchor head portion.
 12. The bone platesystem of claim 9 wherein the projections have substantially flat lowersurfaces that extend over and face the anchor head portion and the setscrew has an annular collar with a substantially flat upper surface thatis arranged below and in overlapping confronting relation with thesubstantially flat lower surfaces of the locking cap projections withthe set screw threadingly engaged to the anchor head portion at thepredetermined axial position.
 13. The bone plate system of claim 9wherein the bone plate includes a bone-facing surface having a lowerprojection extending therefrom and a seating projection spaced from thelower projection such that the bone plate may be positioned on the L5and Si vertebrae with the seating projection resting on the sacralpromontory and the lower projection contacting the sacral bone.
 14. Thebone plate system of claim 9 wherein the locking cap annular wallportion includes a plurality of close-ended slots extending axiallytherealong to resist separation of the locking cap from the anchor headportion due to contact between the locking cap annular wall portion andthe bone plate.
 15. A bone anchor assembly for use with a bone plate,the bone anchor assembly comprising: an elongate bone anchor having ahead portion; a locking cap carried on the anchor head portion; aplurality of projections of the locking cap extending over the anchorhead portion having substantially flat lower surfaces that face theanchor head portion; a locking fastener for being received within theanchor head portion; an annular collar portion of the locking fastener;cooperating cam surfaces of the locking cap projections and the lockingfastener collar portion for shifting the locking cap projectionsradially outward as the locking fastener is driven into the anchor headportion; and a substantially flat upper surface of the annular collarportion that is arranged below and in overlapping confronting relationwith the substantially flat lower surfaces of the locking capprojections when the annular collar portion is advanced axially beyondthe locking cap projections such that the flat confronting surfacesresist locking fastener back out and maintain the bone anchor, lockingcap, and locking fastener in a preassembled configuration.
 16. The boneanchor assembly of claim 15 wherein the anchor head portion includes anannular channel and the locking cap has an annular wall portion and alower cam portion extending radially inward from the annular wallportion with the lower cam portion and anchor head portion configured tohave a snap-fit connection therebetween for receiving the lower camportion in the channel.
 17. The bone anchor assembly of claim 15 whereinthe anchor head portion has an annular wall portion includingcircumferentially spaced depressions extending axially downward andpairs of axially upwardly projecting bosses, the pairs of bosses eachhaving a recess therebetween that is sized and configured to receive thelocking cap projections therebetween and resist rotation of the lockingcap relative to the anchor head portion.
 18. The bone anchor assembly ofclaim 17 wherein the locking cap includes a plurality of slots that arealigned with the depressions of the anchor head portion such that adriver tool may engage the depressions in the anchor head portion anddrive the bone anchor into a bone without turning the locking caprelative to the anchor head portion.
 19. The bone anchor assembly ofclaim 15 wherein the locking cap includes a plurality of upper curvedshoulder portions and axial slots therebetween and the projectionsextend radially inward from the upper curved shoulder portions such thatthe projections shift the upper curved shoulder portions radiallyoutward as the fastener is driven into the anchor head portion.
 20. Amethod of producing a bone anchor assembly, the method comprising:passing an open end of a locking cap over a head portion of a boneanchor; engaging surfaces of the locking cap and anchor head portion toretain the locking cap on the anchor head portion; connecting a fastenerto the bone anchor; and passing an annular collar of the fastener beyondprojections of the locking cap such that the projections retain thefastener on the anchor head and the bone anchor is ready to be driventhrough a bone plate bore and into a bone.
 21. The method of claim 21wherein passing the open end of the locking cap over the head portion ofthe bone anchor includes sliding the open end of the locking cap alongan outer conical surface of the anchor head portion and radiallyexpanding the open end portion.
 22. The method of claim 21 whereinengaging surfaces of the locking cap and anchor head portion includescollapsing the open end of the locking cap into a channel of the anchorhead portion.
 23. The method of claim 21 wherein engaging surfaces ofthe locking cap and anchor head portion includes aligning and seating atleast one locking cap projection between a pair of bosses of the anchorhead portion.
 24. The method of claim 21 wherein passing an annularcollar of the fastener beyond projections of the locking cap includesshifting the locking cap projections radially outward as the fastenerannular collar passes the locking cap projections.
 25. The method ofclaim 21 wherein passing an annular collar of the fastener beyondprojections of the locking cap includes shifting the locking capprojections radially inward and positioning the fastener annular collarin overlapping and confronting relation with the projections.